People suffering from long COVID are reporting a strong smell of fish, sulphur and a sweet sickly odour, as further symptoms of the virus emerge.
The unusual side-effect is known as parosmia - meaning a distortion of smell - and may be disproportionately affecting young people and healthcare workers.
Ear, nose and throat (ENT) surgeon Professor Nirmal Kumar called the symptom "very strange and very unique".
Prof Kumar, who is also the president of ENT UK, was among the first medics to identify anosmia - loss of smell - as a coronavirus indicator in March.
He urged Public Health England to add it to the symptom list months before it became official guidance.
He has now noted that among the thousands of patients being treated for long-term anosmia across the UK, some are experiencing parosmia.
Prof Kumar told Sky News that patients experience olfactory hallucinations, meaning "sense of smell is distorted, and mostly unpleasantly, unfortunately".
He added that it is "really disturbing patients and their quality of life is hugely impacted".
Long COVID is a term to describe the effects of coronavirus that can continue for weeks or months beyond the initial illness.
Describing it as a "neurotropic virus", Prof Kumar explained: "This virus has an affinity for the nerves in the head and in particular, the nerve that controls the sense of smell.
"But it probably affects other nerves too and it affects, we think, neurotransmitters - the mechanisms that send messages to the brain."
He added: "Some people are reporting hallucinations, sleep disturbances, alterations in hearing.
"We don't know exact mechanisms, but we and finding ways to try and help patients recover."
Daniel Saveski, a 24-year-old banker living in London, said he lost his sense of taste and smell for two weeks after contracting coronavirus in March, and has been suffering with parosmia since.
Mr Saveski, from West Yorkshire, said strong-smelling things like bins now have a burning, sulphur-like odour, or smell "like toast".
He added: "It's lessened my enjoyment of food, and it's a bit depressing not being able to smell certain foods."
Lynn Corbett, an administrator for an estate agent, said she was "shocked" to wake up on her 52nd birthday in March with "absolutely no smell or taste".
Ms Corbett, from Selsey in Sussex, said: "From March right through to around the end of May I couldn't taste a thing - I honestly think I could have bitten into a raw onion such was my loss of taste."
She said her sense of smell began to return in June, but "nothing smelled like it should".
"Most things smelled disgusting, this sickly sweet smell which is hard to describe as I've never come across it before."
She said that despite previously being a "coffee addict", the drink now smells "unbearable", as do beer and petrol.
While she's not sure whether she'll ever regain her sense of smell, Ms Corbett said: "I'm okay with it, I just think myself lucky that if I did have coronavirus, which it looks like I did, then I haven't been seriously ill, hospitalised or died from it like so many others."
Charity AbScent, which supports people with smell disorders, is gathering information from thousands of anosmia and parosmia patients in partnership with ENT UK and the British Rhinological Society to aid the development of therapies.
They recommend anyone affected by parosmia to undergo "smell training", which involves sniffing rose, lemon, clove and eucalyptus oils every day for around 20 seconds in a bid to slowly regain their sense of smell.
Prof Kumar said: "There are some promising early reports that such training helps patients."
He added that most people will eventually get their normal sense of smell back.